sufficient to inhibit [a resting cell from enter- 

 ing] nuclear division, but not the process of [a] 

 division that has already begun. Apparently, the 

 intensity of the reaction gradually increases and 

 when it reaches its maximunn, it begins to affect 

 the resting nuclei [derived from divisions allowed 

 to proceed normally after treatment]. Many of 

 them [the mitoses that reappear] are in an ab- 

 normal condition. [Following meiotic irradia- 

 tion]. Stone did not observe any direct, visible 

 effects of irradiation except for clumping and 

 contraction of the chromosomes, which he 

 ascribes to the delay between irradiation and 

 fixation. In material fixed 24 hours after irra- 

 diation, the stages up to metaphase are relatively 

 normal, although in some of the cells fragments 

 are observed at diakinesis and metaphase. From 

 the beginning of first metaphase [anaphase] to 

 second telophase the behavior of the chromo- 

 somes is completely abnormal. These abnor- 

 malities consist of fusion and disintegration of 

 the chromosomes. 



If flower buds are irradiated when the mother 

 cells are in early prophase, then, naturally, the 

 stages of meiosis are normal, except for the 

 appearance of occasional fragments. If the irra- 

 diation is performed two or three months before 

 the beginning of first metaphase, when the tissue 

 is undergoing somatic mitosis, then meiosis also 

 proceeds normally. These observations permit 

 Stone to establish the main difference between 

 the two divisions [i. e. , mitosis and meiosis], 

 which is the difference in the time required to 

 complete the cycle of division. Mitosis proceeds 

 very quickly, taking a few hours, while the ex- 

 tended phases of meiosis require many hours ^ 

 or even days 8 for their completion. Conse- 

 quently, in mitosis the chromosomes have time 

 to complete their division before the induced 

 physiological change attains its highest degree 

 of intensity, while chromosomes in meiosis can 

 be exposed to the induced changes at any stage 

 of division. The division in progress does not 

 stop; the induced change, therefore, affects not 

 only the chromosomes in the resting nucleus, 

 but also the chromosomes at all stages. But 

 only at anaphase do these changes show up in 

 the form of fragments and disintegration of the 

 chromosomes. The division does not stop, but 

 the rate may be slowed down. Stone explains 

 the delay in [reaching] the final stages of meio- 

 sis (in bulbs which were irradiated a month 

 before fixation [when the cells were in very 

 early prophase]) by a slowing down of the divi- 

 sion. The fact that meiosis in bulbs exposed to 

 X rays for 2 or 3 months before proceeds nor- 

 mally indicates that despite the fact that abnor- 

 malities are induced in the somati'" tissue giving 



rise to the pollen mother cells, the latter^ 

 exist for a very short time and are replaced by 

 normal tissue. This is confirmed by the quick 

 reestablishment of normalcy in the morpho- 

 logical and physiological properties of tulips 

 which had been exposed to the influence of irra- 

 diation. A comparison of the results obtained 

 by Stone with those of Goodspeed shows a major 

 disagreement between them. Goodspeed ob- 

 tained abnormal final stages of meiosis due to 

 the effect of X rays on cells of the archesporial 

 tissue and on the pollen mother cells [in early 

 prophase]. Their irradiation in Tulipa caused 

 the final stages to be abnormal.^^ But if the 

 whole meiotic cycle in Nicotiana tabacum is 

 considerably shorter than in Tulipa , Stone's 

 hypothesis is applicable to the results obtained 

 by Goodspeed. 



Mather and Stone (1933) found that the use of 

 X rays as a means of rearrangement of nuclear 

 material has led to very interesting results, 

 especially in Drosophila and Zea mays . Definite 

 types of regroupings were established; why only 

 these types of regroupings should appear was 

 not established. They assume that their obser- 

 vations provide a cytological basis for the 

 appearance of these regroupings as well as 

 certain data concerning the evolution of chromo- 

 some sets. They used three species of Crocus 

 for their experiments: C. olivieri (2n = 6), 

 C. balansae (2n = 6), and C. biflorus (2n = 8). 

 The corms were irradiated for 15 and 30 min- 

 utes; the root tips were fixed after 30 minutes 

 and after 7, 24, 48, and 72 hours. Then the 

 corms were planted in pots and three months 

 later the root tips were again fixed. Cytological 

 examination of these roots permitted the authors 

 to establish that the abnormalities which arise 

 in the chromosomes due to the action of X rays 

 are represented chiefly by two phenomena: 

 fragmentation and fusion of the chromosomes at 

 their ends. These processes lead to all sorts 

 of irregularities: 1) proximal fragments, which 

 are parts of broken chromosomes containing 

 centromeres. They occur very frequently and 

 exhibit perfectly normal behavior. 2) Distal 

 fragments or parts of chromosomes without 

 centromeres. These latter have lost their 

 ability to migrate at anaphase and with each 

 division of the cell there are fewer and fewer of 

 them, and finally they disappear completely. 

 3) Translocations, which arise by fusion of 

 distal segments with whole chromosomes and 

 which are retained after all divisions. (The 

 reciprocal nature of translocations had not yet 



^Text should read "days" according to article by 

 Stone (1933). 



' Text should read "weeks" according to article by 

 Stone (1933). 



*Text should read "the abnormalities" according to 

 article by Stone (1933) . 



'"Editor's note: sentence in text contradictory; 

 Stone (1933) states, "Treatment at this stage in 

 Tulipa , however, yielded normal final stages, with 

 rare exceptions." 



70 



